1. Field of the Invention
This invention generally relates to oscillator circuits. More specifically, the invention provides a precision, low jitter oscillator circuit that is particularly well-suited for generating clock signals used in digital systems, such as a digital hearing aid system. The invention provides utility, however, in any circuit application that requires a precision, low jitter clock source.
2. Description of the Related Art
Oscillators for generating clock signals are known. The two most common types of oscillators are crystal oscillators and ring oscillators. A crystal oscillator is a piezo-electric device that resonates at a particular frequency when a voltage is applied to the crystal. Although some crystals can provide low jitter performance, the mechanical dimensions of the crystal typically limit its applicability in miniaturized circuitry, such as digital hearing aids, implantable bio-electric devices and probes, and/or any other miniature electronic device.
Ring oscillators are generally composed of a plurality of logic gates. The logic gates are typically differential inverters that are configured in a feedback loop in order to provide an oscillation output signal. The cumulative delay through the plurality of differential inverters determines the frequency of the oscillation output signal. These differential inverters, however, typically employ active loads (i.e., transistor devices) in their output stages. The active loads limit the low jitter performance of the oscillator such that to achieve low jitter on the output clock signal, the differential inverters must be operated with a relatively high supply voltage, such as 3 to 5 volts. Miniaturized circuitry, however, typically includes only a low voltage supply, such as a 1.3 volt single battery supply, and thus these known oscillators are of limited utility. Furthermore, in order to modify the frequency of the output clock signal, these active-load differential inverters typically employ an adjustable current source, which further adds to the power consumption of the circuitry. For these reasons, ring oscillators employing differential inverters with active loads have been difficult to integrate into a miniaturized circuit application. (See, for example, A. Hajimire, S. Limotyrakis, and T. H. Lee, “Jitter and Phase Noise in Ring Oscillators,” IEEE Journal of Solid-State Circuits, Vol. 34, No. 6, June 1999, pp. 790–804)
A digital hearing aid system needs an oscillator as a clock reference. The clock must have low jitter to ensure a high signal-to-noise ratio (“S/N”) of both the analog-to-digital (“A/D”) converter, and also the digital-to-analog (“D/A”) converter that are typically included in the hearing aid. Power consumption in this type of system should be minimized since the entire device is typically powered from a 1.3 volt zinc-air hearing aid battery. Other requirements of this type of system include small size and clock precision.